66 Resemblances and Differences Among Living Things 



Our own experience recognizes not only differences but 

 similarities. Yet no individual's experience is ever sufficient 

 to discover that the animals of a given " kind " with which 

 he is familiar grade off through practically imperceptible 

 differences into other groups for which a different name 

 seems appropriate (Fig. 12). 



The Succession of Forms in Time 



Even if we could come to some satisfactory definition 

 of species, so that we could keep each kind of plant or ani- 

 mal distinct and separate from ah other kinds, we should 

 have another difficulty. The supposition that each species 

 arose from a separate act of creation leads us to the idea that 

 such creation was not an act finished at some past time, but 

 one going on continuously. All the facts that we have about 

 the living beings of former times show without any possi- 

 bility of doubt that there lived at various periods plants and 

 animals of kinds that no longer exist. Some of these species 

 came into being at a particular time, increased in numbers 

 of individuals and eventually died out. If we assume that 

 each species arose as a separate act of creation we can under- 

 stand that some, at a particular time, need have had no an- 

 cient ancestors. We can also understand that a species might 

 die out without leaving any descendant. The facts, at any 

 rate, indicate (i) that new species did make their appear- 

 ance from time to time, and (2) that there now exist plants 

 and animals of kinds which are distinct from any for which 

 fossil remains can be found in the oldest or even in some of 

 the more recently formed layers of rocks. It is true, of 

 course, that the fossils represent only a small fraction of the 

 plants and animals that lived at any given period; and of 

 course the fossils which have been found represent probably 

 only a small fraction of the remains that still lie buried in 

 the rocks. 



Further, if we suppose that new species were created 

 from time to time, we should have no reason to expect that 



